A. Field of the Invention
The present invention relates generally to mass spectroscopy and, more particularly, to ion trap mass analysis and analyzers and, further, to specific types of MPIG electrode configurations and methods of use thereof in that context.
B. Discussion of the State of the Art
Advances in modern science have always required the development of new methods of analysis. The evolution of modern instrumentation permits the rapid identification of molecules and allows investigations of structure and reactivity from the atomic scale to the macromolecular scale. Within the last two decades the field of mass spectrometry has become a fundamental method of molecular analysis in the field of biological sciencei,ii,iii,iv. Its utility ranges from simple molecular weight determination to proteomics and protein sequencingv,vi,vii. Although initially limited to small volatile and thermally stable molecules, the real impact of modern mass spectrometry is the wide array of information possible coupled with the high sensitivity of the technique when applied to biologically important molecules.
Investigations of macromolecules by mass spectrometry were historically limited by the inability to produce gas phase ions from large nonvolatile and thermally labile samples. With the development of desorption/ionization techniques, mass spectrometry has become an important tool in the area of biological researchviii,ix,x. The need for mass spectrometry methods capable of analyzing biomolecules has led to the expansion of time-of-flight mass spectrometry (TOF-MS) over the past twenty years. The high sensitivity and high mass range of TOF-MS coupled with external ion sources that produce ions from solution or atmospheric pressure has made TOF-MS a common laboratory technique. While TOF-MS provides high sensitivity mass measurements at high mass, it lacks a direct method of performing linked scans for isolating specific ions for in-depth structural studies. These types of in-depth investigations are typically performed on high performance instruments involving ion trap technology such as quadrupole ion trapsxi, Fourier transform ion cyclotron resonance (FT-ICR) instrumentsxii or more recently, orbitrap analyzersxiii. Although FT-ICR and orbitrap analyzers have the capacity for high mass analysis, the high cost and limited access to these instruments limits their impact on the biological sciences.
Ion trap technologies have long been recognized for their inherent utility in molecular detection and analysis. Ions can be trapped for long periods of time providing the potential for collection and storage of molecules that exist in trace quantities in order to build up a detectable amount for structural studies. The ability to manipulate ions while in the ion trap permits a wide range of investigations including structurally significant tandem mass spectrometry experiments (i.e., MS-MS and MS11). Such mass analysis in an ion trap requires the creation of magnetic or electric fields that can differentiate between ions of different mass. The increased need for this type of analysis of biological samples has led to the production of ion-trap instruments using expensive high field super conducting magnets and specialized shaped electrodes. Because these instruments produce a relatively small area of the required homogeneity, only a limited number of ions can be stored without problems of space charge repulsion. The inherent problem of ion repulsion in these ions traps ultimately limits the dynamic range of the instrumentationxiv.
Orbital ion trapping dates back to 1923 with the introduction of the Kingdon trapxv. The design of this ion trap featured a hollow cylindrical electrode and a thin, wire filament which ran coaxial to the outer cylindrical electrode. The electric field generated by the wire electrode effectively trapped ions in a potential well relative to the reference potential generated by the outer cylinder electrode. When a negative voltage was placed on the center wire electrode relative to the outer cylinder, a potential field was formed that attracted positive charged ions towards the wire electrode. The angular velocity of the ions would cause the particles to orbit the EPG, effectively trapping them in the radial direction. Ions that are accelerated slightly perpendicular to the ion optical axis are captured in the potential field and transported to the detector resulting in orbital trapping of the ions in a radial direction. The addition of positively charged electrostatic electrodes placed at the ends of the cylinder electrode would create an orthogonal potential well that would trap the ions in the axial direction creating an effective ion trap. This basic design was improved in 1985 by Knight by changing the shapes of the end cap electrodes to a chevron geometry. By changing the shapes of the electrodes, the field lines produced created a more effective ion trap. Although both the Kingdon and Knight traps have found application when coupled with mass analyzers, there is no mass dependent frequency created by the electrostatic fields and therefore no method for mass analysis within the trap. This concept was later used by Oakey and McFarlane to increase ion transmission in TOF mass spectrometryxvi. An electrostatic wire electrode was positioned in the drift region of the TOF flight tube creating a potential field in the center which effectively “guided” ions to the detector. Ions that are slightly divergent to the ion optical axis are redirected back towards the detector by the potential field resulting in a dramatic improvement in sensitivityxvii,xviii.
In addition to improved transmission efficiency of ions, Macfarlane demonstrated the utility of the electrostatic ion guide for elimination of neutralsxix and ion eliminationxx. Research performed in my laboratory later demonstrated that selective ion elimination could be accomplished using a pulsed bipolar ion guidexxi,xxii. Recently, this approach was used to create a multi-pass time-of-flight mass spectrometer. In this instrument, ions are effectively trapped in an elongated Kingdon trap by positioning two reflecting electrodes at the extremes of the TOF analyzerxxiii,xxiv. Ions traveling through the drift region between the reflecting fields are continually redirected by the potential field of the ion guide resulting in an enhancement in sensitivity and resolution. In addition, this approach also permits ion selection experiments to be performed by the pulsed ejection of unwanted ions. Although the ion guide effectively traps the ions within the drift region between the reflection fields, the single electrostatic potential generated by the wire ion guide cannot be used with the constantly increasing field of a reflectron instrument. To address this issue, a multi-potential electrode was developed in my laboratory to provide increased ion trapping efficiency in reflecting electric fields. The electrode was created by coating a non-conducting substrate with resistive materials and controlling the voltage at the extremes of the electrode. By varying the resistivity of the surface, the electrode acts as a voltage divider providing a continuum of electric potentials in contrast to the uniform field of the electrostatic wire ion guide. In this manner, a multi-potential ion guide (MPIG) was constructed for use in a reflectron that increased the ion transmission efficiency by an order of magnitudexxv.
Although the ability to mass analyze trapped ions was explored in the early 1950's using an ExB ion trap, the ability to mass analyze trapped ions without the use of a magnetic field was expanded with the development of the Paul quadrupole ion trap in the early 1960's. The Paul ion trap utilized hyperbolic shaped electrodes to create quadrupolar field lines permitting molecular weight determination by the mass dependent effect of an applied rf field. The sympathetic motion of the ions with the oscillating electric field results in a mass selective stability of ions within the trap. The resulting mass dependent stability can be used to either selectively eject ions into a detector or selectively store them for ms-ms type experiments. The flexibility and robust nature of this design has made the quadrupole ion trap a very effective method of mass analysis.
In contrast to the Paul trap which uses oscillating electric fields for mass analysis, the concept of mass selective orbital trapping in an electrostatic ion trap was recently introduced with the development of the orbitrap. The design of the orbitrap improved upon the Knight trap by changing the shape of the center electrodes. The design featured an outer barrel-like electrode and a shaped inner spindle-like electrode. When voltage was then applied between the two electrodes, an electrostatic field was generated capable of trapping ions. Owing to the electrostatic field lines created between the shaped inner electrode and the outer barrel electrode, motion in the z or axial direction is independent of angular and radial motion. Because the axial motion is independent of initial energy and spatial spread of the ions within the trap, the motion can be described as harmonic. This allows for axial frequency to be used for determination of the m/z ratio.
Similar to methods of detection in FT-ICR (Fourier transform ion cyclotron resonance) instruments, detection of ion frequency by image current detection is possible in the orbitrap. By amplifying the induced signal voltage produced of trapped ions as they oscillate, the sum of the image current will include the individual frequencies of ions trapped. This has been achieved in the orbitrap, by splitting the outer electrode and attaching a differential amplifier and detecting an image current. In addition to utilizing detection of an image current, the orbitrap instruments are able to operate in mass-selective instability mode. In this mode, oscillating electric fields or Rf voltage is floated upon the high voltage of the center electrode while the split outer electrodes remained at ground. When Rf voltage is applied to the center electrode at a frequency resonant to axial ion oscillation frequency, the axial component is amplified until the resonant ions are ejected along the axis. By positioning a photomultiplier along this axis, the ejected ions can be detected.
Another example of mass analysis in an electrostatic ion trap was demonstrated by me using a multi-pass reflectron time-of flight (TOF) mass spectrometer (see, e.g., U.S. Pat. No. 6,013,913 to Dr. Curtiss Hanson, which is incorporated by reference herein). Typically reflectrons are included in TOF instruments to focus kinetic energy differences between ions of the same mass thus increasing the mass resolution of the instrument. A TOF system that contains two coaxial reflectrons becomes similar in design to the Kingdon trap with ions trapped in the radial direction by an EPG electrode and axially by the two reflectrons. Ions can be reflected back and forth with the mirrors increasing the net flight length and permitting kinetic energy focusing for enhanced resolution. Similar to the mass instability mode of the orbitrap, ion detection is accomplished by dropping the applied voltage on one of the reflectrons, with mass analysis achieved by the time of flight to the detector.
The efficiency of ion transmission and storage in a multi pass TOF system is limited due to radial dispersion of the ions while in the reflectron region. Because the homogeneous electric field generated by an electrostatic EPG is incompatible with the constantly changing fields needed for a reflectron, there exists no trapping of the ions in the radial direction while in the reflectron field. The lack of the radial trapping field leads to ion dispersion and loss of transmission efficiency. This problem was addressed though the application of a multi-potential ion guide (MPIG). An MPIG is an electrode that is created by coating a non-conducting substrate with resistive materials and controlling the voltage at the extremes of the electrode. By varying the resistivity of the surface, the electrode acts as a voltage divider providing a continuum of electric potentials in contrast to the uniform field of the electrostatic wire ion guide. In this manner, a multi-potential ion guide (MPIG) was constructed for use in a reflectron that produced an electric field that was shaped to match the changing potential of the reflectron. The application of the MPIG in the reflectron region permitted continuous ion trapping in the radial direction while in the reflectron region resulting in increased ion transmission efficiency by an order of magnitude.
Trace chemical analysis is becoming increasingly important in today's society. Compound and molecular identification impacts all areas of industry and environmental monitoring as well as the medical field and law enforcement. As the need for more information about substances has grown, the necessity for sensors capable of providing detailed molecular information has also grown. Although highly sensitive detectors have been developed for identification of specific species or compounds, they are typically large instruments confined to laboratories. There continues to be a growing need for small portable analyzers that can provide information about a wide range of compounds that exist only in trace quantities in the environment that are suitable for work in both the laboratory and in the field.
Over the last decade, mass spectrometry has risen to the forefront of trace molecular identification (xxvi,xxvii,xxviii,xxix,xxx,xxxi). Evidence of its proliferation is seen not only in the news reports of environmental monitoring and law enforcement, but also in popular media where laboratories are often shown using mass spectrometry to identify any unknown. The utility of mass spectrometry is marked by its ability to provide specific structural and molecular identification of unknown compounds from only trace levels of samples. Its combination of high sensitivity as well as powerful specificity makes it the analyzer of choice for many applications. The applications are vast, from drug and explosive testing in law enforcement to pesticide and toxin identification in the environment. Initially, mass spectrometry was an expensive technique that was found only in the most technical laboratories. Today mass spectrometers can be found in almost every analytical laboratory and hospital. As the cost and size of these instruments has decreased, their impact has grown in an increasing range of applications.
The term mass spectrometry refers to the analysis and identification of compounds by measuring their molecular mass. In the simplest sense, the analyzer is similar to a balance, weighing the molecule and evaluating its structure on the basis of its mass. There are many types of mass analyzers because of the large number of methods available for separating and measuring masses of particles. Just as balances for determining the weight of objects have developed over time, so have the instruments for measuring masses of molecules. Since the early 1980's technological advances have led to the development of a number of different instruments for mass spectrometric analysis. These new generation mass spectrometers have increased the sensitivity and versatility of the technique by trapping ions for prolonged periods of time, allowing enhanced chemical study. These ion trap analyzers have typically relied on quadrupolar electric fields (xxxii,xxxiii,xxxiv,xxxv) or crossed electric and magnetic fields (xxxvi,xxxvii) to both contain and analyze the molecules. Although the versatility and performance of these ion trap analyzers make them a valuable technique for trace molecular analysis (xxxviii,xxxix), the high cost of the instruments limits their broader use.
Creation of the electric fields needed to trap and analyze molecules has always relied on producing uniquely shaped electrodes that will provide the desired effect. These complex shaped electrodes are often both difficult and expensive to produce, resulting in high cost and an inability to reduce the size of the instrument which limits the portability and range of implementation of the spectrometer. In my laboratory at the University of Northern Iowa, we developed a new method of producing electrodes in which insulators are coated with semiconductor polymers (xl). By varying the conductivity of the surface of the electrode, it is possible to create complex shaped electric fields through chemical modification rather than physical manipulation of the shape of the electrode. Using this approach, a single chemically modified electrode can be used to create any potential surface desired. This system provides an alternative method of electric field generation and has been used to develop new methods of mass analysis.
The concept of a MPIG is described in detail (called “variable potential ion guide) in U.S. Pat. No. 6,657,190 to Dr. Curtiss Hanson and Paul Trent and is incorporated by reference herein. Using this approach, it is possible to create a user defined electric field by altering the resistivity of the surface of an electrode.
However, the inventor has identified there is room for improvement in the state of the art, and discovered that principles from his prior work can be applied in beneficial ways in the context of ion trap mass analyzers.